You won't find any faster software renderer for Java than what jPCT offers IF (big if!) you are looking for something that renders smooth shaded, textured and depth buffered, perspective correct polygons with sub-pixel accuracy.
That slab thing seems (judging just from the look of the renderings) to do flat shaded, untextured polygons (not sure about the depth buffer...i guess you don't need that for voxels?). If you limit yourself to that, you might be able to get better performance out of it than what you have ATM. It won't be as fast as slab though, because Java is pretty bad in displaying the actual rendering. Copying the image to screen takes more time than it actually should in Java.
Also keep in mind that jPCT already offloads the rendering to multiple cores. You either have to do this too in your software renderer or you have to be multiple times faster to surpass it in terms of performance.